A Point-To-Point Multilink Protocol (MP) is described in standards such as ISO7776 (1988) and Draft Standard RFC1990 (1996). The MP scheme obtains higher transmission bandwidths by establishing multiple separate Point-To-Point Protocol (PPP) links. Once a bundle is established, Internet Protocol (IP) packets are distributed among the different links in the multilink bundle. The data is then reassembled from the different links at a bundle termination point.
Draft Standard RFC1990 is a variation on the basic multilink scheme where large messages are fragmented into multiple packets. The fragmented packets are transmitted through the different links in the bundle. Packets for other smaller messages are then interleaved between the fragmented packets of the larger message. The fragmented packets are then recombined in the correct sequence at the bundle termination point.
Multiclass Multilink (MCMP) is described in Request For Comment 2686 and adds the ability to have multiple sequence number streams, so that packets interleaved with the fragmented packets, such as voice packets, can be given sequence numbers.
The process of fragmenting messages into packets, transmitting the packets over the different links in the multilink bundle, interleaving the fragmented packets with packets from other messages, and reassembling the packets in the correct order is known as Link Fragmentation and Interleaving (LFI). An equivalent FRF.12 protocol is used for fragmentation and interleaving in Frame Relay networks.
Multilink bundles can carry a mix of digital data and real-time traffic, such as voice data. Voice data can be given priority by being assigned to a priority queue that gets priority over other data queues. A multilink load-scheduling algorithm distributes the voice packets in the priority queue among all the different links in the multilink bundle. This Quality of Service (QoS) queuing arrangement is known as Low Latency Queuing (LLQ), where the Priority Queue (PQ) is used for voice, and a Class Based Weighted Fair Queue (CBWFQ) is used for other data.
If the members of the MP bundles use an Asynchronous Transfer Mode (ATM) or Frame Relay (FR) protocol, the layer two ATM or FR network can provide priority to particular virtual circuits in the multilink bundle. This ensures that the traffic on selected virtual circuits receive higher priority than other virtual circuits.
The problem is that there is no way to ensure that high priority traffic on a virtual circuit receives higher priority treatment than data traffic on the same virtual circuit, in the layer-two network. For example, in periods of high network congestion, the layer-2 network will indiscriminately drop both high priority and non-high priority traffic from the non-priority virtual circuits in the multilink bundle. The priority traffic can be interleaved with the non-priority traffic in the multilink bundle. This can increase the latency for the priority traffic.